The latest on the PNAS hybridization scandal

On August 28th, 2009, PNAS published a paper communicated by Lynn Margulis (i.e. it did not go through proper peer-review), by Donald I. Williamson, which began like this.

I reject the Darwinian assumption that larvae and their adults evolved from a single common ancestor. Rather I posit that, in animals that metamorphose, the basic types of larvae originated as adults of different lineages, i.e., larvae were transferred when, through hybridization, their genomes were acquired by distantly related animals.

Williamson [(2009) Proc Natl Acad Sci USA 106:15786–15790] has made the astonishing and unfounded claim that the ancestors of the velvet worms directly gave rise to insect caterpillars via hybridization and that evidence of this ancient “larval transfer” could be found in comparisons among the genomes of extant onychophorans, insects with larvae, and insects without larvae. Williamson has made a series of predictions arising from his hypothesis and urged genomicists to test them. Here, we use data already in the literature to show these predictions to be false. Hybridogenesis between distantly related animals does not explain patterns of morphological and life-history evolution in general, and the genes and genomes of animals provide strong evidence against hybridization or larval transfer between a velvet worm and an insect in particular. [Emphasis added.]

This has led to a comment by Gonzalo Giribet in PNAS on October 30th, 2009:

What remains to test from Williamson’s phylogenetic speculation? Why did the author ignore the weight of phylogenetic evidence that utterly falsifies his claim? Perhaps the most amazing thing from this article is not the bold proposal, but the fact that the author believes that there is a research program behind his claims: ‘‘As an initial trial, it should be possible to attach an onychophoran spermatophore to the genital pore of a female cockroach and see if fertilized eggs are laid’’ (1). I am not sure this can be taken seriously. [Emphasis added.]

And this in turn prompted a reply from Williamson in the same edition (this is not the full reply):

This example is part of my much larger thesis that the basic forms of all larvae were transferred from other taxa, and they all originated as adults (4). Across the animal kingdom, I claim that larvae were acquired from animals at all levels of relationship: bilateral larvae of radial echinoderms originated in an animal in a different superphylum from echinoderms, but most crustacean larvae were acquired from other crustaceans. As yet, no geneticist has carried out tests for larval transfer along the lines that Giribet suggests, but I hope my PNAS article (3) will prompt some of them to do so. Since 2000, several workers have suggested that many planktonic larvae were ‘‘secondarily acquired’’ and have been ‘‘intercalated’’ into the life histories of echinoderms, molluscs, and other phyla (ref. 5 and references therein). These authors do not discuss the sources of these intercalated larvae or mention my work, which does, but they seem to be following in my footsteps, unwittingly, and some distance behind.

(...)

I thank Giribet for drawing attention to my paper (6) that outlines the importance of hybridization in the Cambrian explosion, and in which I claim that there is no cladistic explanation of the origins of phyla. We are indebted to Darwin for his description of a gradual and continual type of evolution, but biologists should also recognize the importance of saltational and sporadic evolutionary processes like symbiogenesis and hybridogenesis. [Emphasis added.]

Wonderful reply, really. He expects researchers to take his idea seriously and actually perform experiments to test it (I am aware that creationist would have a field day with such a remark, thinking all ludicrous claims should of course be tested or else stand as a gaping hole in evolutionary theory), while many better hypotheses could be tested instead. It's not like coming up with hypotheses to test is much a a problem, you know.

Also, you gotta love his delusions of grandeur: "they seem to be following in my footsteps, unwittingly, and some distance behind." Guffaw! It's not the first time I have seen ageing scientists worried sick over their legacy.

The last highlighted sentence about symbiogenesis and hybridogenesis makes it clear why Margulis would let this paper slip through (or pushed it through, rather). An honest comment from her on this whole episode seems warranted at this time.

Pleiotropy comes from the Greek πλείων pleion, meaning "more", and τρέπειν trepein, meaning "to turn, to convert". It designates the occurrence of a single gene affecting multiple traits, and is a hugely important concept in evolutionary biology.

I'm a postdoc at UC Santa Barbara.

All Many aspects of evolution interest me, but my research focus is currently on microbial evolution, adaptive radiation, speciation, fitness landscapes, epistasis, and the influence of genetic architecture on adaptation and speciation.